Rear view mirror adhesive - a 'super glue' product. Clean the glass & the mounting surface of the latch well. Start with a razor blade scraper then clean it with isopropyl alcohol then clean it again. Then clean it again. Once more. Did I mention the mounting area must be clean?
Mark the location for the latch on the outside of the vent window (grease pencil or marking pen). Make sure you have the latch's operation figured out (which way is open & which way is closed). Practice putting the latch in place a couple of times without using the adhesive. Clean the latch & glass again.
Follow the directions for the adhesive. One drop is usually advised. Two drops is not better!
Or do like many Spider owners and leave the latch off. How secure do you think a locked vent window will make your fabric topped Spider?